Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. Commonly used fluid sections usually comprise a plunger pump housing having a suction valve in a suction bore, a discharge valve in a discharge bore, an access bore, and a plunger in a plunger bore, plus high-pressure seals (including plunger packing), etc. FIG. 1 is a cross-sectional schematic view of a typical fluid section showing its connection to a power section by stay rods. A plurality of fluid sections similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid section design schematically illustrated in FIG. 2.
Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element or valve body, whereas the term “valve” as used herein includes the valve body, the valve seat, one or more valve guides to control the motion of the valve body, and one or more valve springs that tend to hold the valve closed (i.e., with the valve body reversibly sealed against the valve seat).
Plunger pump housings are subject to fatigue due to stresses resulting from alternating high and low pressures which occur with each stroke of the plunger cycle. Plunger pump housings typically fail in areas of repetitive stress concentration. For example, fatigue cracks may develop in one of the areas defined by the intersecting suction, plunger, access and discharge bores as schematically illustrated in FIG. 3.
To reduce the likelihood of fatigue cracking in the high pressure plunger pump housings described above, a Y-block housing design has been proposed. The Y-block design, which is schematically illustrated in FIG. 4, reduces stress concentrations in a plunger pump housing such as that shown in FIG. 3 by increasing the angles of bore intersections above 90°. In the illustrated example of FIG. 4, the bore intersection angles are approximately 120°. A more complete cross-sectional view of a Y-block plunger pump fluid section is schematically illustrated in FIG. 5.
Although several variations of the Y-block design have been evaluated, none have become commercially successful for several reasons. One reason is that mechanics find field maintenance on Y-block fluid sections difficult. For example, replacement of plungers and/or plunger packing is significantly more complicated in Y-block designs than in the earlier designs represented by FIG. 1. In the earlier designs, provision is made to push the plunger distally through the cylinder bore and out through an access bore. This operation, which would leave the plunger packing easily accessible from the proximal end of the plunger bore, is impossible in a Y-block design.
A brief review of plunger packing design will illustrate some of the problems associated with packing and plunger maintenance in Y-block fluid sections. FIG. 6 is an enlarged view of the packing in an earlier (but still currently used) fluid section such as that illustrated in FIG. 1. In FIG. 6, the packing and packing brass are installed in the packing box of the fluid section. Note that packing brass is a term used by field mechanics to describe bearing bronze, where the bronze has the appearance of brass.
In the fluid section portion schematically illustrated in FIG. 6, the packing box is an integral part of the fluid section housing; it may also be a separate unit bolted to the fluid section housing. The packing is retained, tightened and adjusted by turning the gland nut. Removing the gland nut, however, does not allow one to remove the packing rings. Because packing rings must block high-pressure fluid leakage past the plunger, they are typically quite stiff, and they remain substantially inaccessible while the plunger (or any piece of it) remains in the plunger bore. FIG. 7 schematically illustrates portions of a plunger pump housing such as that shown in FIG. 5, with components including a gland nut and plunger parts. Note that the distal end of the plunger (i.e., the pressure end) is within the packing box. Note also that the plunger pressure end cannot be rotated for removal until it clears the packing brass. This illustrates the necessity for a two-piece plunger in which the two pieces must be separated as they are individually removed from the plunger bore.
The necessity for a multi-piece plunger in Y-block fluid section housings has not been eliminated by the recent introduction of packing assemblies such as those called “cartridge packing” by UTEX Industries in Houston, Tex. An example of such cartridge packing is schematically illustrated in FIG. 8. Note that removal of the gland nut exposes the packing cartridge housing, which in turn may be fitted with attachment means to allow extraction of the packing cartridge from the packing box (requiring proximal travel of the packing cartridge housing of approximately three to five inches).
This extraction, though, is not practical while a plunger piece lies within the packing box because of the excessive drag of the compressed packing rings on the plunger and packing box walls. Such compression can not be released unless all plunger pieces are removed from the packing box because the packing rings in the above cartridge packing assemblies are pre-compressed when the assemblies are manufactured. Further, any slight misalignment of apparatus used to extract such a cartridge packing assembly tends to cause binding of the (right cylindrical, i.e., not tapered) packing assembly within the (right cylindrical) bore in which it is installed. Analogous difficulties occur if an attempt is made to replace such a cartridge packing assembly while a plunger or part thereof lies in the packing box area. Hence, even if such cartridge packing assemblies were used in Y-block fluid section housings, multi-piece plungers would preferably be used and field maintenance would be correspondingly complicated and expensive.
Thus the Y-block configuration, while reducing stress in a plunger pump housing relative to earlier designs, is associated with significant disadvantages. However, new high pressure plunger pump housings that provide both improved internal access and superior stress reduction are described in copending U.S. patent application Ser. No. 10/288,706, as amended (hereinafter the '706 application), of which the present application is a continuation-in-part. FIG. 9 is a schematic illustration showing examples of structural features disclosed in the '706 application. It includes a right-angular plunger pump housing comprising a suction valve bore (suction bore), discharge valve bore (discharge bore), plunger bore and access bore. The suction and discharge bores each have a portion with substantially circular cross-sections for accommodating a valve body and valve seat with substantially circular cross-sections. Note that the illustrated portions of the suction and discharge bores that accommodate a valve seat are slightly conical to facilitate substantially leak-proof and secure placement of each valve seat in the pump housing (e.g., by press-fitting). Less commonly, the portions of suction and discharge bores intended to accommodate a valve seat are cylindrical instead of being slightly conical. Further, each bore (i.e., suction, discharge, access and plunger bores) comprises a transition area for interfacing with other bores.
The plunger bore of the right-angular plunger pump housing of FIG. 9 comprises a proximal packing area (i.e., an area relatively nearer the power section) and a distal transition area (i.e., an area relatively more distant from the power section) plus a central area between the proximal packing area and the transition area. The proximal packing area comprises a tapered portion for accommodating a corresponding tapered cartridge packing assembly and a threaded portion for accommodating threads of a gland nut. The transition area of the plunger bore facilitates bore interfaces (i.e., reduces stress at bore intersections) at analogous transition areas of other bores as noted above.
Each bore transition area of the right-angular pump housing of FIG. 9 has a stress-reducing feature comprising an elongated (e.g., elliptical or oblong) transverse cross-section having a relatively longer major axis and a (perpendicular) relatively shorter minor axis. Each such cross-section major axis is substantially perpendicular to its respective bore's longitudinal axis and is also perpendicular to a plane that contains (or is parallel to) the longitudinal axes of the suction, discharge, access and plunger bores. Intersections of the bore transition areas are chamfered, the chamfers comprising additional stress-reducing features.
An elongated suction bore transition area, as described in the '706 application, can simplify certain plunger pump housing structural features needed for installation of a suction valve (including its valve spring and valve spring retainer). Specifically, the valve spring retainer of a suction valve installed in such a plunger pump housing does not require a retainer arm projecting from the housing. Nor do threads have to be cut in the housing to position the retainer that secures the suction valve seat. Benefits arising from the absence of a suction valve spring retainer arm include stress reduction in the plunger pump housing and simplified machining requirements. Further, the absence of threads associated with a suction valve seat retainer in the suction bore eliminates the stress-concentrating effects that would otherwise be associated with such threads.
Threads can be eliminated from the suction bore if the suction valve seat is inserted through the suction bore transition area and press-fit into place as described in the '706 application. Following this, the suction valve body can also be inserted through the suction bore transition area. Finally, a valve spring is inserted via the suction bore transition area and held in place by an oblong suction valve spring retainer, an example of which is described in the '706 application and illustrated in FIG. 9. Note that the '706 application illustrates an oblong suction valve spring retainer having a guide hole (for a top-stem-guided valve body), as well as an oblong suction valve spring retainer without a guide hole (for a crow-foot-guided valve body) as shown in FIG. 9. Both of these oblong spring retainer embodiments are secured in a pump housing of the '706 application by clamping about an oblong lip, the lip being a structural feature of the housing (see FIG. 9).
The '706 application also shows how stem-guided valves can be mounted in the fluid end of a high-pressure pump incorporating positive displacement pistons or plungers. This configuration contrasts with conventional well service pumps having both suction and discharge valves that typically incorporate a traditional full open seat design with each valve body having integral crow-foot guides. Crow-foot-guided valves have been found tolerant of the high pressures and repetitive impact loading experienced by valve bodies and valve seats used in well service. But stem-guided valves with full open seats could also be considered for well service because they offer better flow characteristics than traditional crow-foot-guided valves. Stem-guided valves have not been more widely adopted for such use in part because, in a full open seat configuration, stem-guided valves require guide stems on both sides of the valve body (i.e., “top” and “lower” guide stems) to maintain proper alignment of the valve body with the valve seat during opening and closing. Unfortunately, designs incorporating secure placement of guides for both top and lower valve guide stems of suction valves have, before improvements described in the '706 application, been associated with complex components and difficult maintenance.